Chapter 19: Sexual Reproduction and Genetics
THE BENEFITS OF SEX
• Sexual Reproduction Generates Genetic Diversity
• Sexual Reproduction Gives Organisms a Competitive
Advantage in a Changing Environment
Sexual Reproduction Involves both Diploid and Haploid Cells
• Organisms that reproduce sexually are
diploid and contain two sets of
chromosomes, one set from each parent
•
Autosomal chromosomes: all
chromosomes except for sex
chromosomes (X and Y)
• Maternal and paternal versions of each
chromosome are called homologs,
meaning they carry the same sets of
genes (but usually not identical)
• The sperm and egg are called gametes
•
•
Gametes are haploid, meaning they
carry only 1 set of chromosomes from
each parent
Sperm and egg fuse to form a zygote, a
fertilized egg which is once again diploid
and contains both maternal and
paternal chromosomes
Sexual Reproduction Involves both Diploid and Haploid Cells
Germ-line cells and somatic cells carry out fundamentally different functions
• In sexually reproducing animals, diploid germ-line cells,
which are specified early in development, give rise to
haploid gametes by meiosis
• The gametes propagate genetic information into the
next generation
• Somatic cells form the body of the organism and are
therefore necessary to support sexual reproduction, but
they themselves leave no progeny.
• Sometimes gametes can acquire mutations that are
passed down to progeny (germline mosaicism). These
mutations are not present in somatic cells.
Sexual Reproduction Generates Genetic Diversity
• Sexual reproduction produces novel chromosome
combinations (basis of evolution)
• During meiosis, each gamete receives an unique
mixture of maternal and paternal DNA
•
•
The resulting zygote therefore has a combination of
maternal and paternal DNA sequences unique from
either parent
These DNA sequences encoding each gene are called
alleles
• Within a general population, sexual reproduction
and meiosis produces many different alleles in the
population, leading to a diverse “gene pool”
•
•
In any given individual, two copies of the same gene
are likely to be different from one another
This underlies the principal of hybrid vigor
• Thus, the various cycles of sexual reproduction
(diploid, meiosis, haploid, cell fusion) are needed
to break up old combinations of alleles for the
generation of new alleles!
Sexual Reproduction Gives a Competitive Advantage in a Changing Environment
• New genetic combinations (alleles) are generated at random
• May provide advantages or disadvantages to a current environment
• Since environments are unpredictable and change over time,
reshuffling genetic information ultimately produces alleles that
are better suited for the present environment, and thus provide a
competitive advantage
• These alleles are naturally selected for over time as organisms that
are more successful at adapting to their current environment or
more likely to mate
• e.g. advantages for fighting bacteria, viruses, parasites (malaria)
• Advantages for securing sources of food or mate
• Ability to manipulating environment for self-preservation
(intelligence)
• Alleles that do not provide these advantages are naturally selected
against and are gradually eliminated from the population’s gene pool
Key to meiosis is understanding how new allele combinations are generated!
MEIOSIS AND FERTILIZATION
• Meiosis Involves One Round of DNA Replication Followed by Two
Rounds of Nuclear Division
• Duplicated Homologous Chromosomes Pair During Meiotic Prophase
• Crossing-Over Occurs Between the Duplicated Maternal and Paternal
Chromosomes in Each Bivalent
• Chromosome Pairing and Crossing-Over Ensure the Proper Segregation
of Homologs
•
The Second Meiotic Division Produces Haploid Daughter Nuclei
•
Haploid Gametes Contain Reassorted Genetic Information
•
Meiosis Is Not Flawless
•
Fertilization Reconstitutes a Complete Diploid Genome
Meiosis Involves One Round of DNA Replication Followed by Two Rounds of Nuclear Division
N represents the number of
chromosomes in the haploid
nucleus
(2 alleles for each gene)
(1 allele for each gene)
• Mitosis and meiosis both begin with a round of chromosome duplication
• In mitosis, chromosome duplication is followed by a single round of cell division to
yield two diploid nuclei
• In meiosis, chromosome duplication in a diploid germ-line cell is followed by two
rounds of division, without further DNA replication, to produce four haploid nuclei
Meiosis Involves One Round of DNA Replication Followed by Two Rounds of Nuclear Division
• In mitosis, each diploid nucleus produces
two diploid nuclei, which are packaged
by cytokinesis into two diploid cells
• In meiosis, chromosome duplication is
followed by two meiotic divisions to
produce haploid nuclei
•
•
Each diploid nucleus that enters meiosis
produces four haploid nuclei, which are
packaged into haploid gametes by
specialized forms of cytokinesis
Chromosome segregation during meiosis
I and meiosis II is random , so each
haploid gamete has a different mixture
of maternal and paternal alleles
• Mitosis produced two identical diploid
nuclei, whereas meiosis produces
genetically dissimilar haploid cells!
Duplicated chromosomes line up independently during mitosis
• In mitosis, the individual
duplicated maternal (M) and
paternal (P) chromosomes
line up independently at the
metaphase plate before
segregating into daughter
nuclei
• Duplicated alleles are called
sister chromatids
• Ensures equal segregation of
one allele for each gene
from each parent
Duplicated Homologous Chromosomes Pair During Meiotic Prophase
• In meiosis, duplicated maternal and paternal
homologs must pair before lining up at the
metaphase plate
•
•
Time consuming step called meiotic prophase I
Ensures that each haploid cell produced will
receive a sister chromatid from each
chromosome set
• Each pairing forms a structure called a bivalent
•
•
Four sister chromatids stick together until ready
to divide
Pairing depends on interactions between
matching maternal and paternal DNA sequences
• The maternal and paternal homologs separate
during the first meiotic division, and the sister
chromatids separate during meiosis II
Crossing-Over Occurs Between the Duplicated Maternal and Paternal Chromosomes
• During meiosis I, non-sister chromatids in each
bivalent swap segments of DNA. This is called
homologous recombination
• The process begins when protein complexes
produce a double-strand break in the DNA of
one of the chromatids
• These proteins then promote the formation of
a cross-strand exchange with non-sister
chromatids in each bivalent
Crossing-Over Occurs Between the Duplicated Maternal and Paternal Chromosomes
• Following cross-strand exchange, the maternal
and paternal chromatids can physically swap
chromosomal segments (crossing over) at
homologous regions
• This is how you generate new alleles that
contain pieces of maternal and paternal
DNA
• Chromosomes contain recombination
hotspots so some sequences are more likely
to recombine than others (underlies gene
stability)
• Similar to the repair of DNA double-strand
breaks in somatic cells
• these crossing over events are the major source
of genetic variation between sexually
reproducing species
Crossing-Over Occurs Between the Duplicated Maternal and Paternal Chromosomes
cohesin
• The synaptonemal complex facilitates crossing-over
• Helps hold together the paired duplicated homologs in the bivalent
• Aligns homologs to promote strand exchange between non-sister chromatids
• Each chromatid in one bivalent can form a cross-over with either or both of
the non-sister chromatids from the other bivalent
Crossing-Over Occurs Between the Duplicated Maternal and Paternal Chromosomes
• After the synaptonemal complex has
disassembled, the crossover events create a
chiasma between non-sister chromatids in
each bivalent
• Most bivalents contain multiple chiasmata
(avg 2-3/bivalent)
Chromosome Pairing and Crossing-Over Ensure Proper Segregation of Homologs
Chiasmata help ensure proper segregation of
duplicated homologs during the first meiotic
division.
• In metaphase of meiosis I, chiasmata hold the
maternal and paternal homologs together. At
this stage, cohesin proteins also keep the
sister chromatids glued together along their
entire length
• The kinetochores of sister chromatids function
as a single unit in meiosis I, and microtubules
that attach to them point toward the same
spindle pole (chiasmata help resist tension
and position bivalents at metaphase plate)
Spindle pole
Spindle pole
Cohesins degraded
• At anaphase of meiosis I, the cohesins are
suddenly degraded, allowing the homologs to
be separated. Cohesins at the centromere
continue to hold the sister chromatids
together as the homologs are pulled apart.
The Second Meiotic Division Produces Haploid Daughter Nuclei
In meiosis II, as in mitosis, the kinetochores on
each sister chromatid function independently,
allowing the two sister chromatids to be pulled
to opposite poles.
• In metaphase of meiosis II, the kinetochores
of the sister chromatids point in opposite
directions
• At anaphase of meiosis II, the cohesins
holding the sister chromatids together at the
centromere are degraded, allowing
kinetochore microtubules to pull the sister
chromatids to opposite poles
• Sister chromatids for each chromosome line
up, as in mitosis. The L/R orientation can vary,
such that the sister chromatid from one
chromosome pair can segregate with either
sister chromatid from the next chromosome
Haploid Gametes Contain Reassorted Genetic Information
Two kinds of genetic reassortment
generate new chromosome combinations
during meiosis.
• The independent assortment of the
maternal and paternal homologs
during meiosis produces 2n different
haploid gametes for an organism with
n chromosomes
• Here n = 3, and there are 23, or 8,
different possible gametes.
• But the number of possible allelic
recombinations due to crossing over
ensures endless possibilities for
gamete production (unique allelic
sequences). So, in this example, the
true number of possible gametes is
greater than 8.
Meiosis Is Not Flawless
Errors in chromosome segregation during
meiosis can result in gametes with incorrect
numbers of chromosomes
• In this example, the duplicated maternal and
paternal copies of Chromosome 21 fail to
separate normally during the first meiotic
division. As a result, two of the gametes
receive no copy of the chromosome, while
the other two gametes receive two copies.
• Gametes that receive an incorrect number of
chromosomes are called aneuploid gametes.
If one of them participates in the fertilization
process, the resulting zygote will also have an
abnormal number of chromosomes. A child
that receives three copies of Chromosome 21
will have Down syndrome
MENDEL AND THE LAWS OF INHERITANCE
• Mendel Studied Traits That Are Inherited in a Discrete Fashion
• Mendel Disproved the Alternative Theories of Inheritance
• Mendel’s Experiments Revealed the Existence of Dominant and Recessive Alleles
• Each Gamete Carries a Single Allele for Each Character
• Mendel’s Law of Segregation Applies to All Sexually Reproducing Organisms
• Alleles for Different Traits Segregate Independently
• The Behavior of Chromosomes During Meiosis Underlies Mendel’s Laws of Inheritance
• Genes That Lie on the Same Chromosome Can Segregate Independently by Crossing-Over
• Mutations in Genes Can Cause a Loss of Function or a Gain of Function
• Each of Us Carries Many Potentially Harmful Recessive Mutations
Mendel Studied Traits That Are Inherited in a Discrete Fashion
• Gregor Mendel is the father of
genetics
• He pioneered our
understanding of how these
genetic traits are passed to
subsequent generations
• Friar in a Abbey where he
had a pea garden
• His pea plants had many
different physical traits that
could be inherited
• Mendel experimented with
genetics by removing pollen
from males and fertilizing
the female plants, and
studying offspring
Mendel Disproved the Alternative Theories of Inheritance
• Mendel’s experiments began with “truebreeding” plants which produce identical
offspring when allowed to self fertilize
•
Previous “geneticists” had focused on organisms
that varied in multiple traits, so offspring had a
complicated combination of those traits and
could not be easily compared with parents
• Mendel’s approach was unique at the time
•
He studied one trait at a time and would crosspollinate two of his true breeders and record
the inheritance of the chosen trait
• In the first round, true-breeders produce traits
in the F1 generation that match one parent
•
•
At first glance, these results seem to support
the theory of uniparental inheritance
But Mendel went one step further
Mendel’s Experiments Revealed the Existence of Dominant and Recessive Alleles
What happened to the other trait, did the green plant fail to make genetic contributions?
• Next, Mendel allowed the F1 plants to self-fertilize
• 75% had yellow-pea plants and 25% had greenpea plants—so the green pea plant had indeed
passed down genetic material
• This finding extended to other traits besides
color
• These findings led Mendel to propose that unique
hereditary factors govern the inheritance of traits
and the variations
• We now know these to be controlled by
different alleles of the same gene
• The collection of alleles possessed by an
individual, i.e. its genetic makeup, is called
its genotype
Mendel’s Experiments Revealed the Existence of Dominant and Recessive Alleles
What happened to the other trait, did the green plant fail to make genetic contributions?
• Mendel’s findings let him to postulate that
organisms must each inherit two copies of the
hereditary factors
•
•
•
True breeders have two copies of the same factor
(alleles) whereas F1s inherit an unique allele from
each parent
Two copies of the same allele is called homozygosity
Two copies of different alleles is called heterozygosity
• The organisms phenotype depends on the unique
combination of alleles, or whether they are
homozygous or heterozygous for alleles corresponding
to a single gene
•
•
Mendel further hypothesized that some alleles are
dominant over others that are recessive
The dominant allele will always dictate the trait over the
recessive allele, unless two recessive alleles are
inherited (here, yellow is dominant, green is recessive)
Each Gamete Carries a Single Allele for Each Character
• Mendel’s Law of Segregation: Two alleles for each trait
(one from each parent) segregate during gamete
formation and the unite at random at fertilization
• The true-breeding yellow-pea plants produce only Ybearing gametes; the true-breeding green plants
produce only y gametes
• The F1 offspring of a cross between these parents all
produce yellow peas, and they have the genotype Yy.
When these hybrid plants are bred with each other, 75%
of the offspring have yellow peas, 25% have green
• A Punnett square (named after a British mathematician)
allows one to track the segregation of alleles during
gamete formation and to predict the outcomes of
breeding experiments
•
According to the system established by Mendel, capital
letters indicate a dominant allele and lowercase letters a
recessive allele
Mendel’s Law of Segregation Applies to All Sexually Reproducing Organisms
Recessive alleles all follow the same Mendelian
laws of inheritance
• Albinism is a rare condition inherited in a
recessive manner
•
•
The gene that encodes for skin
pigmentation (melanin) is dominant (A)
Albino humans are homozygous
recessive (aa) for variants in the gene
that result in pigment loss
• For most traits or variations, you need
large familial pedigrees with several
generations to accurately predict how
the trait is being inherited
A
a
A
AA
Aa
a
Aa
aa
Mendel’s Law of Segregation Applies to All Sexually Reproducing Organisms
This pedigree shows the risks of first-cousin marriages.
• pedigree for a family that harbors a rare recessive mutation causing deafness
•
•
•
squares represent males, circles are females
the deaf phenotype are indicated by a blue symbol, those that do not by a gray symbol
A black horizontal line connecting a male and female represents a mating between
unrelated individuals, and a red horizontal line represents a mating between blood
relatives (offspring of each mating are shown underneath)
Each of Us Carries Many Potentially Harmful Recessive Mutations
• Different alleles can alter the fitness of an organisms and its ability to thrive in
certain environments. Through natural selection, alleles that provide the greatest
advantages are passed down to subsequent generations
• Many “mutations” are functionally inert and do not cause deleterious
consequences
• Mutations that cause detrimental effects are usually selected against because it
impedes with the survival of the organism and the ability to mate
• Recessive diseases are more complicated: They require two bad copies of the
gene. So healthy individuals with only one bad copy may still be able to mate and
have offspring, perpetuating the existence of the defective gene. Eventually, two
bad copies will be inherited leading to the disorder
• Some deleterious mutations are surprisingly common in the general population
but exist in the heterozygous state
• Eventually, equilibriums are reached, where the rate of new mutations is balanced
by the rate at which mutant alleles are lost through matings that produce
compromised offspring
Mutations in Genes Can Cause a Loss of Function or a Gain of Function
• Mutations in protein-coding genes can affect the protein product in a variety of ways
• Loss of function mutations: reduce or eliminate gene activity or the function of
the encoded protein
• Gain of function mutations: increase activity of a gene or the encoded protein
• Many loss of function mutations only produce phenotypes when recessive because
the functional copy can usually make up for the loss (advantage to having two
chromosomes, so you have a backup copy!)
• Some loss of function mutations can be dominant. For example, some proteins need
to dimerize in order to function, so a dysfunctional protein may interfere with its
functional counterpart (dominant-negative effect)
Mendel’s Law of Segregation Applies to All Sexually Reproducing Organisms
• Mendel’s law of segregation applies to all sexually reproducing organisms
•
•
Dogs are bred to enhance certain phenotypic traits (body size, coat color, head shape,
snout length, ear position, fur, etc)
Scientists have been conducting genetic analyses on scores of dog breeds to search
for the alleles that underlie these common canine characteristics. One gene has been
linked to body size, and three additional genes have been shown to account for coat
length, curliness, and the presence or absence of “furnishings”—bushy eyebrows and
beards—in almost all dog breeds
Alleles for Different Traits Segregate Independently
• A dihybrid (two traits) cross demonstrates that
alleles can segregate independently, which is
known as the law of independent assortment
• Alleles that segregate independently are
packaged into gametes in all possible
combinations
• So the Y allele is equally likely to be packaged
with the R or r allele during gamete formation,
and the same holds true for the y allele
• Thus four classes of gametes are produced in
roughly equal numbers: YR, Yr, yR, and yr
• When these gametes are allowed to combine at
random to produce the F2 generation, the
resulting pea phenotypes are yellow-round,
yellow-wrinkled, green-round, and greenwrinkled in a ratio of 9:3:3:1.
The Behavior of Chromosomes During Meiosis Underlies Mendel’s Laws of Inheritance
• The separation of duplicated homologous
chromosomes during meiosis explains
Mendel’s laws of segregation and
independent assortment
• Independent assortment of the alleles for
seed color, yellow (Y) and green (y), and for
seed shape, round (R) and wrinkled (r),
illustrate how two genes on different
chromosomes segregate independently
• Although crossovers are not shown, they
would not affect the independent
assortment of these traits, as the two genes
lie on different chromosomes.
Genes on the Same Chromosome Can Segregate Independently by Crossing-Over
Genes that lie far enough apart on the same
chromosome will segregate independently
• Because crossover events occur randomly along
chromosomes during prophase of meiosis I, two
genes on the same chromosome will
independently assort if they are far enough
apart.
• There is a high probability of crossovers occurring
in the long region between C/c and F/f. Thus, the
gamete carrying the F allele will wind up with the
c allele as often as it will the C allele
• The A/a and B/b genes are close together, so
there is only a small chance of crossing-over
between them: thus the A allele is likely to be coinherited with the B allele, and the a allele with
the b allele. From the frequency of
recombination, one can estimate the distances
between the genes
•
An example of a crossover that has separated the
C/c and F/f alleles, but not the A/a and B/b alleles